Electroactive polymers are now frequently used in a number of optical devices such as in polymeric light emitting diodes (“PLEDs”) as disclosed In WO 90/13148, photovoltalc devices as disclosed In WO 96/16449 and photodetectors as disclosed in U.S. Pat. No. 5,523,555.
A typical PLED comprises an organic electroluminescent layer located between an anode and a cathode. In operation, holes are injected Into the device through the anode and electrons are injected into the device through the cathode. Holes enter the highest occupied molecular orbital (“HOMO”) of the electroluminescent polymer and electrons enter the lowest unoccupied molecular orbital (“LUMO”) and then combine to form an exciton which undergoes radiative decay to give light. The color of light emitted from the electroluminescent polymer depends on its HOMO-LUMO bandgap.
An electron transport material is commonly used to assist in transport of electrons from the cathode to the LUMO of the electroluminescent polymer and thus increase device efficiency. Suitable electron transport materials are those having a LUMO level falling between the LUMO level of the electroluminescent polymer and the workfunction of the cathode. Similarly, a hole transporting material having a HOMO level falling between the workfunction of the anode and the HOMO level of the emissive material is commonly used. For example, WO 99/48160 discloses a blend of a hole transporting polymer, an electron transporting polymer and an electroluminescent polymer. Alternatively, the electron transporting functionality and the emissive functionality may be provided by different blocks of a block copolymer as disclosed in WO 00/55927.
A focus In the field of PLEDs has been the development of full color displays for which red, green and blue electroluminescent polymers are required—see for example Synthetic Metals 111-112 (2000), 125-128. To this end, a large body of work has been reported in the development of electroluminescent polymers for each of these three colors with red, green and blue emission as defined by PAL standard 1931 CIE coordinates.
A difficulty encountered with blue electroluminescent polymers is that their lifetime (i.e. the time taken for brightness to halve from a given starting brightness at fixed current) tends to be shorter than that of corresponding red or green materials. One of the factors that has been proposed as contributing to the more rapid degradation of blue materials is that their LUMO levels, and consequently the energy level of the charged state following injection of an electron into the LUMO, tends to be less deep than those of corresponding red or green materials. It is therefore possible that materials comprising these lower electron affinities are less electrochemically stable and therefore more prone to degradation.
For simplicity, a full color display will preferably have the same cathode material for all three electroluminescent materials. This results in the further problem that the energy gap between the LUMO and the workfunction of the cathode for a typical blue electroluminescent polymer is greater than that for a typical red or green electroluminescent polymer. This may contribute to lower efficiency.
Clearly, assisted electron injection into blue electroluminescent polymers is desirable, however the choice of electron transporting material is constrained by the fact that the emissive material is generally that with the smallest bandgap. This limitation is particularly restrictive in the case of blue electroluminescent polymers since the bandgap required for blue emission is the largest of red, green and blue.
Chains of fluorene repeat units, such as homopolymers or block copolymers comprising dialkylfluorene repeat units, may be used as electron transporting materials. In addition to their electron transporting properties, polyfluorenes have the advantages of being soluble in conventional organic solvents and have good film forming properties. Furthermore, fluorene monomers are amenable to Yamamoto polymerization or Suzuki polymerization which enables a high degree of control over the regioregularity of the resultant polymer.
One example of a polyfluorene based polymer Is a blue electroluminescent polymer of formula (a) disclosed in WO 00/55927:
wherein w+x+y=1, w≧0.5, 0≦x+y≦0.5 and n≧2.
In this polymer, chains of dioctylfluorene, denoted as F8, function as the electron transport material; the triphenylamine denoted as TFB functions as the hole transport material and the bis(diphenylamino)benzene derivative denoted as PFB functions as the emissive material.
WO 94/29883 discloses use of electron withdrawing groups, particularly nitrile groups, as substituents on electroluminescent polymers for the purpose of reducing the barrier to electron injection between a high workfunction electrode and the electroluminescent polymer. This document only teaches use of such substituents on poly(arylene vinylenes).
J. Poly. Sci. Part A: Polym. Chem. Vol. 39 (2001) discloses a polymer of repeat units of formula (b):

This disclosure describes use of fluorinated sidechains as a means of decreasing interchain interactions that have been reported to cause aggregation of polyfluorenes and contains no discussion of using such electron deficient substituents as a means to increase electron affinity. This polymer is disclosed as showing no photoluminescence.
There are disclosures of diphenylfluorenes wherein the phenyl group carries substituents, however these substituents are electron donating as measured by their Hammett sigma constants. For example, WO 00/22026 discloses a homopolymer having a repeat unit of formula (c):

Also disclosed in this document are copolymers of (c) with dialkylfluorene repeat units and with triarylamine repeat units. Asymmetric substitution of the 9-position of fluorene is described for the purpose of avoiding polymer aggregation; this document contains no teaching of 9-substituents used for the purpose of enhanced electron injection of the fluorene backbone. Similarly, WO 99/20675 discloses a 1:1 copolymer of 9,9-di-n-octylfluorene and 9,9-di(4-methoxyphenyl)fluorene and WO 01/62822 discloses a polyfluorene with triarylamine 9-substituents.
JP 10095972 discloses a molecule of formula (e):

This is disclosed as an emissive material of the type known as “small molecules” rather than polymers as described hereinbefore. This molecule is used In conjunction with a separate, electron transporting molecule. The use of fluorine substituents on the phenyl ring is not described for the purpose of increasing electron affinity of the fluorene ring; fluorine substituents are merely one of a large number of possible substituents for the phenyl ring disclosed in this document.
It is an object of the invention to provide a high electron affinity material that is capable of functioning as an electron transport material for a blue electroluminescent material. For the reasons explained above, such a material would also be capable of functioning as an electron transport material for a red or green material. Furthermore, such material may, as a result of Its large HOMO-LUMO bandgap, be used as a blue electroluminescent material.